Method for making a perfected medical model on the basis of digital image information of a part of the body

ABSTRACT

A method is set fourth for making a perfected medical model on the basis of digital image information of a part of the body. According to which this image information of a part of the body is converted, by means of what is called the rapid prototyping technique and thus with a processing unit (4) and a rapid prototyping machine (5), into a basic model (9) of which at least a part perfectly shows the positive or negative form of at least a portion of the part of the body. At least an artificial functional element (10) with a useful function is added to the basic model (6) as a function of the digital information and possibly as a function of additional external information.

FIELD OF THE INVENTION

The invention concerns a method for making a medical model on the basisof digital image information of a part of the body, according to whichthis image information of a part of the body is converted, by means ofwhat is called the rapid prototyping technique and thus with aprocessing unit and a rapid prototyping machine, into a basic model ofwhich at least a part shows the positive or negative form of at least aportion of the part of the body.

BACKGROUND OF THE INVENTION

By rapid prototyping technique should be understood all techniqueswhereby an object is built layer by layer or point per point by addingor hardening material (also called free-form manufacturing). The bestknown techniques of this type are: stereo lithography and relatedtechniques, whereby for example a basin with liquid synthetic materialis selectively cured layer by layer by means of a computer-controlledelectromagnetic beam; selective laser sintering, whereby powderparticles are sintered by means of an electromagnetic beam or are weldedtogether according to a specific pattern; or fused deposition modelling,whereby a synthetic material is fused and is stacked according to a linepattern.

The digital image information can be provided by a computer tomographyscanner.

The model produced up to now according to the above-mentioned technique,can be a model which is an exact copy of the part of the body, forexample a piece of bone, and upon which a surgery operation can bepracticed, or it can be a prosthesis which fits perfectly to the part ofthe body.

However, the models produced up to now, including three-dimensionalimages, do not take advantage of all the information contained in theimage information. They form a perfect copy of the part of the body, butthey do not contain any additional functional elements.

Such models which are exact copies of real structures are for exampleproduced from medical images with the technique disclosed in the article"Integration of 3-D medical imaging and rapid prototyping to createstereolithographic models" from T.M.BARKER et al., published in"Australasian Physical & Engineering Sciences in Medicine", vol. 16, no.2, June 1993, pages 79-85.

Scanner data are transformed to a suitable format in a computer and theimages are processed as a volume of voxels. The object is segmentedprior to the meshing of the object surface and the creation of thestereolithographic model. The obtained model cannot be used forregistration, this is finding back a position on the patient.

Functional elements, such as an opening indicating the place anddirection for boring, can be added manually, but not as a function ofthe image information. At the time when these models are made, the greyvalue data of the image information are lost. However, these grey valuedata contain clinical data which are important for the use of themodels. Such clinical data are for example the muscles and tendons whichhave to be taken into account when designing a prosthesis. These musclesand tendons are visible in the images, but not in the three-dimensionalmodel, nor when working with segmented contours/surfaces inCAD-applications.

How to color selected elements of a three-dimensional object such as ananatomic model, prepared by irradiation techniques for example bystereolithography, is disclosed in EP-A-0 535 984, but this documentdoes however neither disclose nor suggest to add artificial functionalelements to the model for registration purposes, this is for transposingthe pre-surgical planning or simulation to the surgery.

The manipulation of digital image data during the preparation of asurgical operation, for example, is known as such. It is possible, forexample, to determine the position and direction of an implant on theimages or to simulate surgeries. However, there is no connection withreality and, by lack of reference, these prepared image data cannot beused in practice. The image information is not used to the full extent.

As for the application of dental implants, attempts have already beenmade to use teeth of a provisional prosthesis as a reference. Thisprovisional prosthesis is made on the basis of a mould. With areconstruction by means of computer tomography scanner images on thebasis of planes in which the bone is clearly visible, what is called adental scan, one can see whether the position and the angle of theprovisional teeth are correct in relation to the underlying bone, andone can make corrections. However, this is a time-consuming method.

Sometimes, a template is made on the basis of the mould and thistemplate is used during the surgery. Only surface data are used hereby,so that part of the information of the dental scan remains unused.

Another method consists in making a model of the jaw by means of therapid prototyping technique and to make a template on the basis of thismodel which is used during the surgery. The information of the digitalimage of the teeth (the dental scan) cannot be used either with thismethod.

SUMMARY OF THE INVENTION

The invention aims to remedy these disadvantages and to provide a methodfor making a perfected medical model on the basis of digital imageinformation of a body part whereby the image information can beoptimally used and can be put to use in practice.

This aim is reached according to the invention as at least oneartificial functional element with a useful function is added to thebasic model, as a function of the digital image information in the formin which all medical data are visible, this is in the grey value imageinformation, before segmentation, the useful function of the functionalelement being an indication of a physical parameter, such as a position,a direction, a length or an angle which are important during surgery orthe shape of a bone elevation.

External information coming from the medical user may be added to theimage information, the artificial functional element being also as afunction of this additional external information.

By subsequently converting the image with the additional information ininformation for the control of a rapid prototyping machine, there is afeedback of the medical data to reality and a perfected model isobtained which does not only have the shape of a certain part of thebody, such as a ragged bone shape, and thus provides a perfectreference, but which also contains artificial elements which are addedas a function of the image information and of possible new additionalinformation, and which have a useful function.

The functional element is added as a function of the digital imageinformation in the form in which all medical data are visible.

Such a form of the image information consists of the grey valueinformation.

In a peculiar embodiment of the invention, the information on the basisof which the functional element is determined is processed factually inthe perfected model by means of a voxel oriented computer system.

Via contour generation (segmentation/interpolation), one can switch fromimage processing to, for example, stereo lithography.

New information may be added from outside to determine the functionalelement, such information must then also be presented as voxels orcontours.

The functional element with a useful function can be a shape, a color ora texture.

The method can be used in numerous applications.

Thus, it can be usefully applied in combination with the already appliedcomputer aided surgery simulation, whereby bone segments are cut andmoved at a certain angle and over a certain distance. With the help ofthe method, templates and jigs can be made which provide a perfectreference on the one hand and indicate angles and movements on the otherhand.

The method can also be used for the preparation of tooth implants,whereby the perfected medical model is a template and the functionalelement is an opening or notch on the place where drilling is required,or for making a knee prosthesis, whereby the basic model is a metal basewhich can be joined to a sawn off tibia or femur and whereby thefunctional elements are orientation pins and/or fastening pins whichstand on the base and which position and/or fix a prosthesis. Also anactual prosthesis can be made according to the method, part of whichfits perfectly to existing bone and another part of which forms thefunctional element with a prosthetic function.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better explain the characteristics of the invention, thefollowing preferred embodiments of a method for making a perfectedmedical model on the basis of digital image information of a part of thebody are given as an example only without being limitative in any way,with reference to the accompanying drawings, in which:

FIG. 1 shows a general block diagram of a method for making a perfectedmedical model according to the invention;

FIG. 2 schematically shows how a perfected medical model is made on thebasis of the image;

FIG. 3 schematically shows how another form of a perfected medical modelis made according to the method of the invention;

FIGS. 4, 5, 6, 7 and 8 schematically show how yet other forms ofperfected medical models for other applications can be made according tothe invention.

DESCRIPTION OF THE INVENTION

As is schematically represented in FIG. 1, images 3 are made of a partof the body of a patient 1 by means of a computer tomography scanner 2or any other digital image processing unit such as a Magnetic ResonanceImage machine, which thus contain digitized medical information.

Instead of converting these images in, for example, a three-dimensionalimage or a dental scan and subsequently either making a model by meansof rapid prototyping or processing the images, the image data will befirst processed in a processing unit 4, after which a perfected model 6is made with these processed digitized image data by means of rapidprototyping with a rapid prototyping machine 5. Use can be made for thisoperation of a visual three-dimensional image 17 or a dental scan 18which is derived in the usual manner from the images 3. Thisthree-dimensional image 17 and this dental scan 18 are represented inFIG. 1 by means of a dashed line.

What is characteristic is that the model 6 can be used in reality on thepatient 1 or in other words that the cycle represented in FIG. 1 iscompleted. In this figure, everything that is situated under the dashedline 19 represents reality, and everything that is situated above it isimmaterial information.

The processing or preparation includes the manipulation of medicaldigital image data, possibly with additional digital information fromoutside, in such a way that an artificial, functional element 10 with auseful function is added to the produced basic model 9.

The processing of existing and possibly new information or the "design"is carried out with a voxel-oriented system in the processing unit 4,i.e., by means of voxels or contours, whereby voxels or groups of voxelsare indicated in the images 3. A voxel is a three-dimensional pixel andthus represents a cube. The grey value data of the voxels can be used toobtain still higher resolutions and accuracies. The processing unit 4which controls the rapid prototyping machine can help during theprocessing by carrying out operations on those voxels which are standardoperations in three-dimensional image processing, such as thresholding(segmentation on the basis of grey values), three-dimensional reduction,expansion, region growing, boolean operations such as adding andsubtracting, projections, etc.

If external technical elements are added, for example coming from a CADsystem, these elements must be represented as voxels or contours aswell. This can be easily done by means of cross section and shadingalgorithms.

After the interactive processing of the image information (for examplerotations, translations, etc.), it is possible to go back to theoriginal CAD data to obtain a higher resolution and accuracy of thefunctional element.

FIG. 2 shows an enlarged representation of one of the images 7 with greyvalues, derived in the processing unit 4 in the form of voxels from theimages 3 of a bone 20 produced by the scanner 2. Through processing inthe processing unit 4 are made negative images 8 in voxel form which fitperfectly to the images 7 and thus to the bone. Moreover, the image 11of a functional element 10 is provided in voxel form in the images 8.The images 8 coincide with a reference part which forms a negative basicmodel 9 which fits perfectly to the bone, which basic model 9, togetherwith the functional element 10, forms the perfected model 6.

In FIG. 2 is represented by means of a dashed line 21 the boundarybetween what is reality (underneath it) and what is image information(above it), whereas what is situated above the dashed line 22 isrepresented enlarged and in voxel form.

When providing the image 11 of the functional element 10 in voxel form,one can take into account all medical information contained in theimages 7. Via stereo lithography, the images 8 having the images 1 1 ofthe artificial functional element 10 on top, are converted in thethree-dimensional, factual, perfected model 6 which can be placed as atemplate on the bone of the patient 1 during a surgery and which fitsperfectly to it. The useful function of the artificial functionalelement 10 can then be put to use. The information of the scanner 2 andthe information of the position and direction of the artificialfunctional element 10 based upon it, are in this way used to the fullestextent and translated into reality.

In order to pass from the information of the processing unit 4 to therapid processing technique, one can proceed as follows:

The information or data set, consisting of voxels and contours, of theprocessing unit 4 is converted into a set of contours per layer height.This is done by means of a screen which is finer than the screen of theoriginal images 3, since the rapid prototyping techniques have a higherresolution than the scanner 2. In order to obtain this finer screen, useis made of the grey value information in the images 3. Thus, a pixel orvoxel can partly belong to the perfected model 6 and partly not. Thisphenomenon is known as partial volume effect. When there are only twomaterials in one pixel or voxel, a contour line can be calculated inbetween the pixels by means of interpolation, as described by B.Swaelens and others in "Medical Applications of Rapid PrototypingTechniques", p. 107-120 of "Proceedings of the Fourth InternationalConference on Rapid Prototyping, Dayton, Ohio, Jun. 14-17, 1993". Thishigher resolution is important to make the designed model fit well ontothe part of the body. Once the contours per layer are calculated, theyare interpolated in the third dimension up to the layer height which issuitable for the rapid prototyping technique. This layer height isusually significantly lower than the scan distance.

Another method consists in converting the above-mentioned data set intoa surface description with, for example, one of the usual formats suchas triangle format. (STL). Such descriptions are used to calculatesections which are made by the rapid prototyping machine 5. Here also,it is possible to work with sub-voxel resolution.

According to a third method, the medical data or digital information isconverted from the processing unit 4 to a CAD system. This is againapproached by means of a surface description and by calculating thesections. It is possible to further add elements in the CAD system, butnot as a function of the image information.

The artificial functional element 10 with a useful function can be ashape, a color, a texture or another characteristic element. The usefulfunction of the element 10 can be the indication of a place where, adirection in which, a length over which, or an angle at which, one mustcut, saw or drill; it can also be a point of attachment, the filing ofan existing defect, a prosthetic function or an identification.

In the embodiment represented in FIG. 2, the perfected model 6 is atemplate and the artificial functional element 10 is an opening whichindicates the position and direction for the boring bit of a boringmachine. The basic model 9 forms a reference part. The thickness of thebasic model 9 at the location of the opening determines the depth ofhole.

The method can be used for the preparation of tooth implants. Theposition and the orientation of the implants, both in relation to thebone and in relation to the teeth, is very important. First, a dentalscan is made. Thanks to computer-aided preparations, the thickness,position, direction and length of an implant can be well planned. Bymaking a template according to the invention as represented in FIG. 2,it is not only possible to match the planned size and length of theimplant in reality, but also directly the position and direction. Areference part is formed by the basic model 9 which fits perfectly tothe bone and an element 10 which forms a guide for the boring bit withwhich the hole for the implant is drilled and which determines theposition, direction and depth of the hole.

Instead of directly making a negative perfected model 6, a positivemodel 13-14 of the bone can be made in the above-described manner, butcontaining information regarding the position, direction and depth ofthe drill hole to be made in the form of protrusions 14 as representedin FIG. 3. A basic model 9 is made as a reference part only afterward,for example manually, with openings as functional elements 10, as anegative mould of the positive model 13-14 as is represented in FIG. 4.

Another application resides in the production of a membrane for bonegeneration, whereby this membrane can form the reference part or basicmodel 9 and the artificial functional element 10 is a notch or incisionas represented in FIG. 5. First, a positive intermediate model 15 ismade on the basis of the images 3 of the scanner 2, via stereolithography, consisting of a basic model 9 and the required boneelevation 16 as a first artificial functional element 10. Whereas,according to known methods, the bone elevation is determined byrealizing the elevation in reality in a radiographically visiblematerial, prior to the scanning, the elevation is calculated accordingto the invention by the processing unit 4 and imported in the medicalinformation derived from the grey value data, either departing from anideal bone shape stored in a memory of the processing unit 4 orinteractively.

A second artificial functional element 10 can be possibly provided,namely a place indication, for example in the shape of a notch, therewhere the implant should come. This can be either done through theagency of the user or automatically by means of a computer according toa stored program. In any case, it is preferably provided as a functionof the grey value data in a dental scan.

From the intermediate model 15 is made a perfected model 6 in the shapeof a membrane by making a mould on the basis of the intermediate model15 and by shaping a foil in the mould into a membrane. Just like theintermediate model 15, the membrane is provided with a notch as anartificial functional element 10 which has as a function to indicate theplace of the implant.

In the case where the implant is provided together with the membrane,reference marks or sutures can be provided as artificial functionalelements 10 in the above-described manner to position the membrane inthe space where the bone will grow later.

Another application of the method according to the invention consists inmaking prostheses.

With a knee prosthesis, the sliding surface of both the femur and thetibia must be replaced by sawing away a piece and by replacing thispiece by a prosthesis. Hereby, it is important that the prosthesis fitscorrectly to the bone, especially on the side of the tibia, since thereis only a thin wall of strong cortical bone there to support theprosthesis. When the prosthesis is too large, protruding edges form aproblem.

In the first place, an incision is indicated in voxel form in the images7, there where the tibia or femur should be sawn. A first negative model6 is made in the above-described manner which fits perfectly to the bone20, but which protrudes all round this bone 20 with an edge which is cutoff by the incision. This edge then forms an artificial functionalelement 10 which serves as a guide for the saw with which the incisionis sawn during the surgical operation.

The voxels above the incision are removed in the processing unit 4 and abase 12 is designed here as a reference part or basic model 9 upon whichorientation pins are provided as artificial functional elements 10 bythe processing unit 4. On the basis of this design is made, for exampleby means of stereo lithography and casting, a real model 6 which fitscorrectly to the remaining part of the bone 20 and which is providedwith artificial functional elements 10 which are oriented in the rightmanner.

The base 12 can be designed such that it penetrates partly into the bone20, and especially also partly surrounds the bone on the outside, asrepresented in FIG. 8. This largely increases the strength.

On the basis of the negative model 6 which forms the sawing template,and taking into account the thickness of the base and the position ofthe artificial functional elements 10, a positive model can be made ofthe prosthesis itself. One can hereby depart from the real model 6 ofthe sawing template or preferably from the digital information thereofin the processing unit 4 and calculate the prosthesis with the latter tofinally transform it via rapid prototyping in a real prosthesis. Thisprosthesis will also be provided with artificial functional elements 10which are complementary to those provided on the base 12.

Instead, a standard prosthesis can be provided on the base 12, wherebythe artificial functional elements 10 on the basic model 9 formed by thebase 12 need to be provided in this case as a function of complementaryelements of the standard prosthesis.

A hip prosthesis can be made in an analogous manner which fits perfectlyto the femur shaft on one side and which contains a technical part withfunctional elements on the other side upon which the artificial femurhead can be placed. In the images 7 can be indicated the ideal lengthand the direction.

A prosthesis fitting perfectly to an existing structure on one side andbearing a technical part on the other side which has a prostheticfunction can also be used for dorsal vertebra. Grey value data, forexample regarding the position of the nerves, can be used in theprocessing unit 4 for the design of the prosthesis.

We claim:
 1. In a method of making a medical model from digitalinformation corresponding to a part of a human body wherein the digitalinformation is used to generate information for rapid prototyping of themodel, the improvement comprising:adding to the digital information datarelated to at least one artificial functional element having a usefulfunction related to a physical parameter and based on image informationin the form in which all medical data are visible and including greyvalue image information, before segmentation; carrying out the rapidprototyping of the model using the digital information with the addeddata whereby the medical model includes said artificial functionalelement.
 2. The method according to claim 1, including, as part of saidadding to the digital information step, adding external information froma source separate from the digital information.
 3. The method accordingto claim 2, wherein said external information is used in the form ofvoxel or contour information.
 4. The method according to claim 1,including processing at least said artificial image data used inproducing said model using a voxel oriented computer system.
 5. Themethod according to claim 1, wherein said functional element is selectedfrom the group consisting of shape, color and texture.
 6. The methodaccording to claim 1, including rapid prototyping the model in the formof a reference part contoured to match a part of the body; and in saidstep of adding data related to at least one artificial functionalelement, adding data related to a guide for an instrument to be used onthe body part, whereby the reference part will contain such guide. 7.The method according to claim 1, wherein said medical model comprises areference part for use in a dental implant preparation procedure, andsaid image data is derived at least in part from a dental scan.
 8. Themethod according to claim 1, wherein said model corresponds to a portionof bone on which additional bone is to be generated to a preselectedheight and includes as the artificial functional element the additionalheight corresponding to the additional bone to be generated; and usingthe model to form a metal foil and membrane including said additionalheight.
 9. The method according to claim 8, including using the imagedata to form an added functional element of a notch in the modelcorresponding to the location of an implant to be placed in the bonebefore using the model to form said metal foil and membrane.
 10. Themethod according to claim 1, wherein the medical model is a positivemodel made by rapid prototyping using the digital information and theimage data to make a positive mold containing the functional element;and then making a negative mold of the part of the human body with thefunctional element by using the positive mold.